1. Field of the Invention
The present invention is directed to ink jet printing systems, and in particular to drop-on-demand ink jet printing systems having printheads with heater elements.
2. Description of the Related Art
Ink jet printing systems can be divided into two types. The first type is a continuous stream ink jet printing system and the second type is a drop-on-demand printing system.
In a continuous stream ink jet printing system, ink is emitted in a continuous stream under pressure through at least one orifice or nozzle. The stream is perturbed so that the stream breaks up into droplets at a fixed distance from the orifice. At the break-up point, the droplets are charged in accordance with digital data signals and passed through an electrostatic field which adjusts the trajectory of each droplet in order to direct the ink droplets to a gutter for recirculation or to a specific location on a recording medium.
In a drop-on-demand ink jet printing system, a droplet is expelled from an orifice directly to a position on a recording medium in accordance with digital data signals. A droplet is not formed or expelled unless the droplet is to be placed on the recording medium. Because the drop-on-demand ink jet printing system requires no ink recovery, charging or deflection, such system is much simpler than the continuous stream ink jet printing system. Thus, ink jet printing systems are generally drop-on-demand ink jet printing systems.
Further, there are two types of drop-on-demand ink jet printing systems. The first type uses a piezoelectric transducer to produce a pressure pulse that expels a droplet from a nozzle. The second type uses thermal energy to produce a vapor bubble in an ink-filled channel to expel an ink droplet.
The first type of drop-on-demand ink jet printing system has a printhead with ink-filled channels, nozzles at ends of the channels and piezoelectric transducers near the other ends to produce pressure pulses. The relatively large size of the transducers prevents close spacing of the nozzles, and physical limitations of the transducers result in low ink drop velocity. Low ink drop velocity seriously diminishes the tolerances for drop velocity variation and directionality and impacts the system's ability to produce high quality copies. Further, the drop-on-demand printing system using piezoelectric transducers suffers from slow printing speeds.
Due to the above disadvantages of printheads using piezoelectric transducers, drop-on-demand ink jet printing systems having printheads which use thermal energy to produce vapor bubbles in inkfilled channels to expel ink droplets are generally used. A thermal energy generator or heater element, usually a resistor, is located at a predetermined distance from a nozzle of each one of the channels. The resistors are individually addressed with an electrical pulse to generate heat which is transferred from the resistor to the ink.
The transferred heat causes the ink to be super heated, i.e., far above the ink's normal boiling point. For example, a water based ink reaches a critical temperature of 280.degree. C. for bubble nucleation. The nucleated bubble or water vapor thermally isolates the ink from the heater element to prevent further transfer of heat from the resistor to the ink. Further, the nucleating bubble expands until all of the heat stored in the ink in excess of the normal boiling point diffuses away or is used to convert liquid to vapor which, of course, removes heat due to heat of vaporization. During the expansion of the vapor bubble, the ink bulges from the nozzle and is contained by the surface tension of the ink as a meniscus.
When the excess heat is removed from the ink, the vapor bubble collapses on the resistor, because the heat generating current is no longer applied to the resistor. As the bubble begins to collapse, the ink still in the channel between the nozzle and bubble starts to move towards the collapsing bubble, causing a volumetric contraction of the ink at the nozzle and resulting in the separating of the bulging ink as an ink droplet. The acceleration of the ink out of the nozzle while the bubble is growing provides the momentum and velocity to expel the ink droplet towards a recording medium, such as paper, in a substantially straight line direction. The entire bubble expansion and collapse cycle takes about 20 microseconds (.mu.s). The channel can be refired after 100 to 500 .mu.s minimum dwell time to enable the channel to be refilled and to enable the dynamic refilling factors to be somewhat dampened.
FIG. 1 is an enlarged, cross-sectional view of a conventional heater element design. The conventional heater element 2 comprises a substrate 4, an underglaze layer 6, a resistive layer 8, a phosphosilicate glass (PSG) step region 10, a dielectric isolation layer 12, a tantalum (Ta) layer 14, addressing and common return electrodes 16, 18, an overglaze passivation layer 20, and a pit layer 22. The actual heater area is determined by the length L.sub.R of the resistive material. However, the effective heater area is determined by the distance L.sub.E between the inner slanted walls of the overglaze passivation layer. In another conventional heater element design (not shown), the side walls of the overglaze passivation do not overlap the side walls of the PSG step region, and the effective heater area is determined by the distance between the inner side walls of the PSG step region. Because there is a relatively large difference L.sub.D between the actual heater area and effective heater area, the heat generated at the unused heater areas is lost. Further, the overglaze passivation layer 20 or PSG step region 10 alone prevents exposure of the ionic and corrosive ink to the addressing and common return electrodes and/or resistor ends.
It is generally recognized in the ink jet technology that the operating lifetime of an ink jet printhead is directly related to the number of cycles of vapor bubble expansion and collapse that the heater elements can endure before failure. Further, after extended usage, the heater robustness, i.e., the printhead's ability to produce well defined ink droplets, is degraded. Heater failures and degradation of heater robustness are due to extended exposure of the heater elements to high temperatures, frequency related thermal stresses, large electrical fields and significant cavitational pressures during vapor bubble expansion and collapse. Under such environmental conditions of the heater elements, the average heater lifetime is in the high 10.sup.7 pulse range, i.e., number of ink droplets produced, with the first heater failure occurring as low as 3.times.10.sup.7 pulse range.
Further, the bulk of all heater failures does not occur on the resistors 8 which vaporize the ink, but rather occurs near the junction between the resistor 8 and electrodes 16, 18. Specifically, during the collapse phase of the vapor bubble, large cavitational pressures of up to 1000 atm. impact the regions near the PSG step region 10 and overglaze passivation layer 20 of the heater. The large cavitational pressures result in attrition damage to the tantalum (Ta) layer 14 and dielectric isolation layer 12 and also attrition damage, i.e., notch damage, to the overglaze passivation layer 20 covering the PSG step region 10. Moreover, the overglaze passivation layer 20 alone protects the electrodes 16, 18 from the ionic ink, which is corrosive. Eventually, a hole in the Ta layer 14, dielectric isolation layer 12 and/or passivation layer 20 allows the ionic and corrosive ink to contact the heater at the electrodes 16, 18 to cause degradation of heater robustness and hot spot formation and eventually to heater failures.
Moreover, the heater failures are exacerbated by the problem of obtaining good conformal coverage of the Ta layer 14 over the PSG step region 10. The problem of obtaining good conformal coverage has been corrected by using an extra processing step to taper which consequentially extends the heater lifetime into the low 10.sup.8 pulse range. However, heater failures are still located at the PSG step region 10 and/or the overglaze passivation layer 20, and the cost of fabrication is increased by an extra processing step to obtain good conformal coverage.
Various printhead design approaches and heater constructions are disclosed in the following patents to mitigate the vulnerability of the heaters to cavitational pressures, but none of the patents discloses a heater design which removes the failure prone overglaze passivation layer 20 and/or PSG step region 10 from the region of final bubble collapse so that the PSG step region 10 and overglaze passivation layer 20 are no longer subject to the cycles of vapor bubble expansion and collapse and to the ionic and corrosive ink.
U.S. Pat. No. 4,951,063 to Hawkins et al. discloses a thermal ink jet printhead improved by a specific heating element structure and method of manufacture. The heating elements each have a resistive layer, a high temperature deposited plasma or pyrolitic silicon nitride thereover of predetermined thickness to electrically isolate a subsequently formed cavitational stress protecting layer of tantalum thereon. Such a construction lowers the manufacturing cost and concurrently provides a more durable printhead.
U.S. Pat. No. 5,041,844 to Deshpande discloses a thermal ink jet printhead having an ink channel geometry that controls the location of the bubble collapse on the heating elements. The ink channels provide the flow path between the printhead ink reservoir and the printhead nozzles. In one embodiment, the heating elements are located in a pit a predetermined distance upstream from the nozzle. The channel portion upstream from the heating element has a length and a cross-sectional flow area that is adjusted relative to the channel portion downstream from the heating element, so that the upstream and downstream portions of the channel have substantially equal ink flow impedances. This results in controlling the location of the bubble collapse on the heating element to a location substantially in the center of the heating elements.
U.S. Pat. No. 4,532,530 to Hawkins discloses a carriage type bubble ink jet printing system having improved bubble generating resistors that operate more efficiently and consume lower power without sacrificing operating lifetime. The resistor material is heavily doped polycrystalline silicon which can be formed on the same process lines with those for integrated circuits to reduce equipment costs and achieve higher yields. Glass mesas thermally isolate the active portion of the resistor from the silicon supporting substrate and from the electrode connecting points so that the electrode connection points are maintained relatively cool during operation. A thermally grown dielectric layer permits a thinner electrical isolation layer between the resistor and its protective ink interfacing tantalum layer and thus increases the thermal energy transfer to the ink.
U.S. Pat. No. 4,774,530 to Hawkins discloses an improved printhead which comprises an upper and lower substrate that are mated and bonded together with a thick insulative layer sandwiched therebetween. One surface of the upper substrate has etched therein one or more grooves and a recess, which when mated with the lower substrate, will serve as capillary filled ink channels and an ink supplying manifold, respectively. Recesses are patterned in the thick layer to expose the heating elements to the ink, thus placing them in a pit and to provide a flow path for the ink from the manifold to the channels by enabling the ink to flow around the closed ends of the channels, thereby eliminating the fabrication steps required to open the groove closed ends to the manifold recess so that the printhead fabrication process is simplified.
U.S. Pat. No. 4,835,553 to Torpey et al. discloses an ink jet printhead comprising upper and lower substrates that are mated and bonded together with a thick film insulative layer sandwiched therebetween. A recess patterned in the thick layer provides a flow path for the ink from the manifold to the channels by enabling the ink to flow around the closed ends of the channels and increase the flow area to the heating elements. Thus, the heating elements lie at the distal end of the recesses so that a vertical wall of elongated recess prevents air ingestion while it increases the ink channel flow area and decreases refill time, resulting in an increase in bubble generation rate.
U.S. Pat. No. 4,935,752 to Hawkins discloses an improved thermal ink jet printhead using heating element structures which space the portion of the heating element structures subjected to the cavitational forces produced by the generation and collapsing of the droplet expelling bubbles from the upstream interconnection to the heating element. In one embodiment, this is accomplished by narrowing the resistive area where the momentary vapor bubbles are to be produced so that a lower temperature section is located between the bubble generating region and the electrode connecting point. In another embodiment, the electrode is attached to the bubble generating resistive layer through a doped polysilicon descender. A third embodiment spaces the bubble generating portion of the heating element from the upstream electrode interface, which is most susceptible to cavitational damage, by using a resistive layer having two different resistivities.
U.S. Pat. No. 4,638,337 to Torpey et al. discloses an improved thermal ink jet printhead for ejecting and propelling ink droplets along a flight path toward a recording medium spaced therefrom in response to the receipt of the electrical input signals representing digitized data signals. The recess walls containing the heating elements prevent the lateral movement of the bubbles through the nozzle and therefore the sudden release of vaporized ink to the atmosphere, known as blow out which causes ingestion of air and interrupts the printhead operation.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.